1. Field of the Invention
The present invention relates to polishing apparatuses, and more particularly to a polishing apparatus for accurately measuring the amount of a workpiece which is polished.
2. Description of the Related Art
A known polishing apparatus for polishing a metal, a ceramic, and a semiconductor material has a configuration as shown in FIG. 6.
The polishing apparatus is for polishing the upper and lower surfaces of workpieces W at the same time, and comprises an upper wheel 1 for pressing the workpieces W and a lower wheel 2 for supporting the workpieces W. The upper wheel 1 and the lower wheel 2 are coaxially arranged with each other. A plurality of carries 3, which performs a sun-and-planet rotation while holding the workpieces W, is arranged along the circumferential direction of the upper wheel 1 and the lower wheel 2 and between these two wheels.
The upper wheel 1 is vertically moved by an air cylinder 7 attached to a stationary support member 6. The upper wheel 1 has a substantially spherical holder 1a, formed in the upper middle thereof, for holding a spherical pressure head 8 which is disposed at the bottom of the air cylinder 7.
The pressure head 8 has an electrical micrometer 10 attached thereto as a displacement-detection means for detecting the relative displacement between the upper wheel 1 and the lower wheel 2. The electrical micrometer 10 has a main unit 10a which is fixed to the pressure head 8 and a probe 10b which serves as a displacement-detection rod and which is expandable with respect to the main unit 10a. 
The lower wheel 2 has a short cylindrical shape and has a substantially cylindrical wheel drive shaft 12 coaxially fixed thereto. In addition, the lower wheel 2 is rotatably supported by a bearing 13 and has gear teeth 12a which are formed around the outer periphery of the bottom portion of the lower wheel 2. The gear teeth 12a engage with a gear 14 which is directly connected to a wheel drive motor 15.
The wheel drive shaft 12 has a carrier drive shaft 18, coaxially arranged therein and supported by a bearing 19, for rotating and revolving the carriers 3. The carrier drive shaft 18 has gear teeth 18a which are formed around the outer rim periphery of the bottom thereof and which engage with a gear 20 directly connected to a first carrier drive motor 21. The upper part of the carrier drive shaft 18 is enlarged in diameter to form a diameter-enlarged portion 18b. The diameter-enlarged portion 18b has a reference table 22. The reference table 22 is formed at the center of the upper surface of the diameter enlarged portion 18b in a projecting manner so as to be integral therewith, and against which the probe 10b of the electrical micrometer 10 abuts. The diameter-enlarged portion 18b also has a large number of inner pins 23 which are formed along the outer rim of the upper surface thereof and which engage with a gear-like toothed portion 3a formed along the outer rim of each of the carriers 3.
Outside the lower wheel 2 and the wheel drive shaft 12 fixed thereto, an outer ring 25 supported by a bearing 26 is arranged in a manner coaxial with the lower wheel 2 and the wheel drive shaft 12 so as to rotate and revolve the carriers 3 around the same. The outer ring 25 has gear teeth 25a which are formed around the outer periphery of the bottom portion thereof and which engage with a gear 27 directly connected to a second carrier drive motor 28. The outer ring 25 also has a large number of outer pins 29 which are arranged along an inner rim of the upper surface thereof and which engage with the toothed portion 3a. 
The inner pins 23 and the outer pins 29 function as a sun gear and an inner gear, respectively, so that the first carrier drive motor 21 and the second carrier drive motor 28 rotate synchronously with each other.
When the known polishing apparatus having the foregoing configuration polishes the Workpieces W, the carriers 3 holding the workpieces W are arranged on the lower wheel 2, and the toothed portion 3a formed around the outer rim of the carrier 3 are arranged to engage with the inner and outer pins 23 and 29. Subsequently, the air cylinder 7 is activated for the pressure head 8 to press the upper wheel 1. Thus the workpieces W are sandwiched and pressed between the upper wheel 1 and the lower wheel 2.
In this state, while an abrasive slurry is interposed between the upper wheel 1 and the workpieces W and between the lower wheel 2 and the workpieces W, the wheel drive motor 15 is activated for rotation, and also the first carrier drive motor 21 and the second carrier drive motor 28 are activated for synchronous rotation. Then, the wheel drive shaft 12 and the lower wheel 2 are driven for rotation by the wheel drive motor 15. Also, the carrier drive shaft 18 and the inner pins 23 fixed thereto are driven for rotation by the first carrier drive motor 21, and the outer ring 25 and the outer pins 29 fixed thereto are driven for rotation by the second carrier drive motor 28.
Thus, the inner pins 23 and the outer pins 29, both engaging with the toothed portion 3a formed along the outer rim of the carrier 3, allow the carrier 3 holding the workpiece W to rotate and revolve around the upper and lower wheels 1 and 2 in the circumferential direction thereof. Thus, the carrier 3 performs a sun-and-planet motion, allowing both the upper and lower surfaces of the workpiece W to be polished by utilizing a relative difference in speeds between the upper wheel 1 and the upper surface of the workpiece W and between the lower wheel 2 and the lower surface of the workpiece W.
Polishing the upper and lower surfaces of each of the workpieces W leads to displacement of the upper wheel 1, resulting in a gradual reduction in the distance between the upper wheel 1 and the lower wheel 2. Thus, when the amount of expansion and contraction of the probe 10b abutting against the reference table 22 changes, the main unit 10a of the electrical micrometer 10 outputs detection signals in accordance with the change in expansion and contraction. Then, a controller (not shown) determines whether or not the thickness of the workpieces W agrees with a predetermined target value on the basis of the detection output from the electrical micrometer 10. When the thickness of the workpieces W reaches the target value, the motors 15, 21, and 28 are stopped thus completing the polishing of the workpieces W.
In the known polishing apparatus, the reference table 22 is fixed at the top of the carrier drive shaft 18, and the carrier drive shaft 18 is configured separately from the lower wheel 2 by the bearing 19. With this arrangement, a shaky motion of the bearing 19 or the carrier drive shaft 18 in the axial direction thereof prevents a change in expansion and contraction of the probe 10b from accurately following the relative displacement between the upper wheel 1 and the lower wheel 2, thereby causing a detection error of the amount of polishing of the workpieces W.
That is to say, detection of the relative displacement between the lower surface of the upper wheel 1 and the upper surface of the lower wheel 2 is required in order to measure an accurate amount of polishing of the workpieces W. Since the carrier drive shaft 18, to which the lower wheel 2 is fixed, is configured separately from the lower wheel 2, a slight shift of the carrier drive shaft 18 along the axial direction thereof caused by, e.g., a shaky motion of the bearing 19 and the like leads, to a change in expansion and contraction of the probe 10b. As a result, the change in expansion and contraction of the probe 10b does not accurately follow the relative displacement between the upper wheel 1 and the lower wheel 2, thereby giving rise to an error in detecting the relative displacement.